High-Sulphidation Deposits of Gold

Imagine a process so intense that it turns solid rock into a sponge-like structure, paving the way for precious metals like gold to gather in its porous framework. This is the fascinating story of high-sulphidation gold deposits, where scorching, acidic fluids from the depths of the Earth transform rocks into treasure chambers.

What are high-sulphidation deposits?

High-sulphidation deposits are formed when hot magma releases fluids rich in gases. These fluids mix with groundwater, creating strong acids that dissolve the surrounding rock. The remaining silica-rich rock can form a sponge-like structure, which is ideal for gold and copper deposits to form.

How do these deposits grow?

As the acidic fluids move away from the fault, they become less acidic and alter the surrounding rocks. This creates distinct zones of altered minerals, often in a shell-like pattern. The shape and size of these deposits depend on the distribution of the silica-rich rock.

High-sulphidation systems originate when volcanic gases, rich in compounds like sulfur dioxide (SO₂) and hydrogen chloride (HCl), rise directly from molten magma. When these gases encounter groundwater, they form highly corrosive acids capable of dissolving surrounding rocks, leaving behind a silica-rich skeleton called “vuggy silica.” These sponge-like formations then serve as traps for gold and copper-rich fluids, which precipitate their metals within the porous rock. The size and shape of these deposits are defined by how far these acidic fluids spread, sometimes creating large, low-grade deposits ideal for bulk mining.

Alteration Minerals in High-Sulphidation Systems

But the process doesn’t stop there. As the acidic fluids move away from their source—often a fault—they gradually become neutralized by the rocks they encounter. This creates distinct layers of mineral alteration, with each layer reflecting the fluid's changing chemistry. At the fault’s core, you find vuggy silica. As you move outward, the minerals change progressively to quartz-alunite, kaolinite-dickite, illite, and finally, chlorite-rich rocks at the edges. These transitions create dramatic, color-rich landscapes, often spanning areas as large as 100 square kilometers.

Low-Sulphidation Deposits

By contrast, low-sulphidation gold deposits form in a slower, more intricate process. Here, fluids interact extensively with groundwater, cooling and diluting as they rise. This prolonged interaction neutralizes the acidity, allowing quartz to precipitate in veins. Over time, pressure builds beneath these quartz-sealed faults, leading to sudden ruptures and explosive boiling events. These eruptions deposit high-grade gold and silver in narrow, banded quartz-adularia veins, often over vertical intervals of 300 to 600 meters.

Key differences between high- and low-sulphidation deposits

Acidity: High-sulphidation deposits are more acidic, while low-sulphidation veins are less acidic.

Gold grade: Low-sulphidation veins often have higher gold grades.

Deposit size: High-sulphidation deposits can be larger, while low-sulphidation veins are typically narrower and more compact.

In conclusion, high-sulphidation and low-sulphidation systems offer a fascinating glimpse into the complex processes that shape our planet's geology. By understanding these processes, we can unlock the secrets of gold deposits and uncover the hidden treasures of the Earth.